In the Advanced Liquid Metal Reactors (ALMR), a reactor core of fissionable fuel is submerged in a hot liquid metal, such as liquid sodium, within a reactor vessel. The liquid metal is used for cooling the reactor core, with the heat absorbed thereby being used to produce power in a conventional manner.
A known version of an ALMR plant (shown in FIG. 1) has a concrete silo 8 which is annular or circular. The silo is preferably disposed underground and contains concentrically therein an annular containment vessel 2 in which is concentrically disposed a reactor vessel 1 having a nuclear reactor core 12 submerged in a liquid metal coolant such as liquid sodium. The annular space between the reactor and containment vessels is filled with an inert gas such as argon. The reactor and containment vessels are supported or suspended vertically downward from an upper frame 16, which in turn is supported on the concrete silo 8 by a plurality of conventional seismic isolators 18 to maintain the structural integrity of the containment and reactor vessels during earthquakes and allow uncoupled movement between those vessels and the surrounding silo.
Operation of the reactor is controlled by neutron-absorbing control rods 15 which are selectively inserted into or withdrawn from the reactor core. During operation of the reactor, it may be necessary to shut down the fission reaction of the fuel for the purpose of responding to an emergency condition or performing routine maintenance. The reactor is shut down by inserting the control rods into the core of fissionable fuel to deprive the fuel of the needed fission-producing neutrons. However, residual decay heat continues to be generated from the core for a certain time. This heat must be dissipated from the shut-down reactor.
The heat capacity of the liquid metal coolant and adjacent reactor structure aid in dissipating the residual heat. For instance, heat is transferred by thermal radiation from the reactor vessel to the containment vessel. As a result, the containment vessel experiences an increase in temperature. Heat from the containment vessel will also radiate outwardly toward a concrete silo spaced outwardly therefrom. These structures may not be able to withstand prolonged high temperatures. For example, the concrete making up the walls of the typical silo may splay and crack when subjected to high temperatures.
To prevent excessive heating of these components, a system for heat removal is provided. One of the heat removal systems incorporated in the ALMR is entirely passive and operates continuously by the inherent processes of natural convection in fluids, conduction, convection, and thermal radiation. This safety-related system, referred to as the reactor vessel auxiliary cooling system (RVACS), is shown schematically in FIG. 1. Heat is transported from the reactor core to the reactor vessel 1 by natural convection of liquid sodium. The heat is then conducted through the reactor vessel wall. Heat transfer from the reactor vessel outside surface to the colder containment vessel 2 across the argon-filled gap 3 is almost entirely by thermal radiation. An imperforate heat collector cylinder 5 is disposed concentrically between the containment vessel 2 and the silo 8 to define a hot air riser 4 between the containment vessel and the inner surface of the heat collector cylinder, and a cold air downcomer 7 between the silo and the outer surface of the heat collector cylinder. Heat is transferred from the containment vessel 2 to the air in the hot air riser 4. The inner surface of heat collector cylinder 5 receives thermal radiation from the containment vessel, with the heat therefrom being transferred by natural convection into the rising air for upward flow to remove the heat via air outlets 9. Heat transfer from the containment vessel outer surface is approximately 50% by natural convection to the naturally convecting air in the hot air riser 4 and 50% by radiation to the heat collector cylinder 5.
Heating of the air in the riser 4 by the two surrounding hot steel surfaces induces natural air draft in the system with atmospheric air entering through four air inlets 6 above ground level. The air is ducted to the cold air downcomer 7, then to the bottom of the concrete silo 8, where it turns and enters the hot air riser 4. The hot air is ducted to the four air outlets 9 above ground level. The outer surface of heat collector cylinder 5 is covered with thermal insulation 5a (see FIG. 2) to reduce transfer of heat from heat collector cylinder 5 into silo 8 and into the air flowing downward in cold air downcomer 7. The greater the differential in temperature between the relatively cold downcomer air and the relatively hot air within the riser, the greater will be the degree of natural circulation for driving the air cooling passively, e.g., without motor-driven pumps.
The above description applies to normal reactor operation and shutdown heat removal when the sodium within the reactor vessel is at its normal level 10. In accordance with the foregoing ALMR concept, the reactor vessel and its closure function as the primary coolant boundary. A steel dome located above the reactor closure functions as the primary containment above the closure elevation. Below the reactor closure elevation, the containment vessel functions both as a guard vessel (for leak protection) and as the containment.
The ALMR containment has been shown to be effective against all design basis events and most beyond design basis (BDB) events and is considered to meet and exceed present U.S. licensing requirements. However, it is possible for significant radiological releases to occur under a postulated BDB event in which both the reactor and containment vessels fail. If leaks should develop in both the reactor vessel 1 and the containment vessel 2, the sodium level may drop as low as the double vessel leak level 11 (see FIG. 1). Under such conditions, atmospheric air can come in direct contact with radioactive sodium. Thus, there is the potential for major sodium fires and the escape of radioactive products directly to the atmosphere through the RVACS air inlet and outlet ducts. In addition, the RVACS will be rendered inoperative. This loss of cooling capability would result in heat-up and a slow (five-day) sodium boil-off followed by a core melt-through, which in turn would be followed by a more severe radiological release. These are the major disadvantages with the known ALMR concept.